The present invention is related to an improved process for the manufacture of NMC cathode materials. More specifically, the present invention is related to an improved process for the formation of precursors of lithium metal oxide comprising at least one of nickel, manganese and cobalt salts. Even more specifically, the present invention is related to a process for making precursors of lithium metal oxide requiring low water consumption and recycle of key reactants.
One of the more promising cathode materials for batteries is an oxide comprising various ratios of nickel, manganese and cobalt, such as those referred to in the art as NMC's, wherein an NMC is general represented by the chemical formula:Li2−x−y−zNixMnyCozO2 
wherein x+y+z≤1; and wherein the formula is represented in stoichiometric balance with the understanding that the lithium is mobile and functions as the charge carrier into and out of the cathode as is known in the art.
The process of forming lithium metal oxides includes the formation of a powder comprising salts of the metals followed by calcining of the powder to achieve the oxide in a crystallographic ordered lattice. The unit cells of the crystallographic ordered lattice comprise layers and the lithium can migrate into and out of the layers. There are two primary ways of forming the powder, or precursor. The traditional approach is to intimately mix salts of the metals to form a homogeneous mixture. The homogenous mixture can be formed by many techniques including physical mixing of the solids, co-precipitation, sol-gel and the like, each of which is characterized by the formation of a mixture of metal salts with the choice of technique partially determined by the desired particle size and degree of homogeneity both of which are thought to impact the properties of the ultimate oxide even though quantification of the benefits is difficult to ascertain. Techniques which rely on the mixing of metal salts to form a powder, and preferably a homogenous powder, are characterized by the formation of an amorphous mixture of separate salts.
A modern technique has recently come to the fore as a significant improvement over the mere mixing of salts. The modern technique, referred to in the art as complexometric or complexecelle formation, forms ordered crystalline precursors of metal salts instead of an intimate mixture of powders. The complexometric method relies on carefully controlled precipitation conditions to precipitate an ordered precursor comprising salts of the metals ultimately incorporated in the lithium metal oxide. By way of a non-limiting example, a precursor for forming a lithium metal oxide with equal proportions of nickel, manganese and cobalt would be in the form of an ordered lattice comprising an equal molar concentration of a nickel salt, a manganese salt and a cobalt salt. While not limited to theory, it is hypothesized that by having an ordered lattice of metal salts, as opposed to a mixture of powdered metal salts, the metal migration during the calcining is more efficient thereby allowing the ordered lattice of oxides to have fewer dislocations, fewer crystalline impurities or fewer inactive phases even though this has proven difficult to quantify. Oxides formed from the precursor prepared by the complexometric method have proven to be advantageous with regards to their properties as a cathode in a battery.
The complexometric method, which relies on balancing the solubility of metal salts to precipitate the metal salts in an ordered lattice, requires copious amounts of water and therefore the cost of the process, though advantageous over solid state methods, limits the manufacturing scale achievable within a reasonable space and with reasonable resources as the water must be removed prior to calcining. Removing large volumes of water is neither cost effective nor conducive to a large scale process. Furthermore, the process utilizes materials, such as ammonia or ammonium hydroxide, for pH control which increases the complexity in a manufacturing environment as the ammonia must be removed and either disposed of or recycled neither of which is conducive to environmental stewardship or effective manufacturing practice.
The conventional complexecelle method, as applied to the formation of a precursor for a lithium nickel manganese nickel oxide (NMC), will be described with reference to the flow chart of FIG. 1. In FIG. 1, water and lithium carbonate (A) are introduced to a mixer (Ma). Carbon dioxide (B) is introduced to mixer (Ma) thereby forming an aqueous solution of lithium bicarbonate (C) in accordance with reaction Scheme 1:Li2CO3+CO2+H2O→2LiHCO3  Scheme 1.
The lithium bicarbonate is introduced to reactor (Ra) and metal acetate (D) is metered in thereby forming metal carbonate and lithium acetate. Ammonia (E) is introduced to maintain pH leading to the mixture (F) in accordance with reaction Scheme 2:LiHCO3+MAc2+NH3MCO3+LiAc+NH4Ac  Scheme 2
wherein Ac represents acetate.
Mixture (F) is then separated in separator (Sa), into a liquid stream (G) comprising a large volume of water, possibly metals complexed by ammonia, residual acetates, etc. thereby representing a waste stream of high volume, per mole of solids obtained. The solid component (H), from the separator (Sa), comprises primarily the precursor comprising metal carbonates in an ordered lattice and lithium acetate.
The present invention provides a method of complexometric formation wherein the volume of water required, as a function of the oxide precursors formed, is minimized due to recycling and the inventive method allows for a near continuous flow operation wherein most components not incorporated in the final product are maintained within a steady state manufacturing loop for subsequent reuse.